Known headrest height adjustment devices are described in DE 196 31 843 A1 (FIGS. 2–3) and DE198 11 688 C1 (FIG. 3). The known headrest height adjustment devices adjust a height of a headrest that performs in conjunction with the sliding movement of a seat body in a longitudinal direction of a vehicle. According to the known headrest height adjustment devices, a user (e.g., a driver) can manually adjust a height of a headrest individually.
Because the known headrest height adjustment devices described in DE 196 31 843 A1 and DE198 11 688 C1 include a mechanism for adjusting a height of a headrest that performs in conjunction with the sliding movement of the seat body as well as a mechanism for manually adjusting a height of a headrest individually, construction of a headrest height adjustment device has become complicated.
Further, with the construction of headrest height adjustment devices described in DE 196 31 843 A1 and DE198 11 688 C1, the tension of a cable is used for operating the headrest in either an upward or downward direction, and the biasing force of a spring is used for operating in the opposite, downward or upward, direction. Thus, the space required for positioning the mechanism used for operating the headrest in an upward direction and downward becomes substantial. In other words, the total amount of space is increased by the amount of space required for positioning the spring used for operating the headrest in either an upward or downward direction. In particular, with the construction described in, DE 196 31 843 A1 because the mechanism for adjusting a height of the headrest individually is itself operated in upward and downward directions, a requirement arises for space in which the locus for the upward and downward directions operation can be released. On the other hand, with the construction described in DE198 11 688 C1, sufficient space is required for covering a joint portion of a double pipe which is configured so as to expand and contract.
Thus, it is extremely difficult to install the known headrest height adjustment devices described in DE 196 31 843 A1 and DE198 11 688 C1 inside a seatback of a general design. More particularly, as shown in FIG. 7, a seatback 91 for a vehicle seat of a general design is configured so as incline towards the back of a vehicle in a longitudinal direction. On the other hand, the operational direction of a headrest 92 is determined to be an approximately vertical direction of the vehicle in order to maintain a predetermined constant distance between the headrest 92 and a human body that is occupying the seat. Further, in order to secure space for the feet F of an occupant of a rear seat, the seatback 91 includes a back 91a that is hollowed out in the most forward direction possible. With the foregoing configurations, when the headrest 92 is operated to the maximum downward position possible, it becomes impossible to secure a sufficient distance between an end 93a of a stay 93 and the back 91a (i.e., design surface). This restriction limits the space that is required for the mechanisms in the operational direction of the headrest 92 in upward and downward directions, that is in an extending direction of the stay 93.
Further, with the known headrest height adjustment devices described in DE 196 31 843 A1 and DE 198 11 688 C1, in order to use a mechanism for adjusting the height of a headrest that performs in conjunction with the sliding movement simultaneously with a mechanism for adjusting the height of the headrest individually, a lock release mechanism for releasing the headrest is connected to an operational system (e.g., slide lever) of a seat slide mechanism. This arrangement is for purposes of stabilizing the adjustment operation of a height of the headrest in conjunction with the sliding movement and involves resetting the height of the headrest to an initial state that has been individually adjusted at the time of a sliding operation. Accordingly, in cases where a headrest height adjustment device is applied to a motor driven (electrically driven type) seat slide mechanism, a dedicated actuator is required for releasing the headrest in synchronization with the timing of activating a motor.
Further, in order to prevent noise caused by oscillation while a vehicle is moving, a clearance between the stay 93 of the headrest and a sliding guide support 94 (shown in FIG. 7) is generally set at a minimum level, and thus sliding resistance between the stay 93 and the sliding guide support 94 becomes substantial. Accordingly, with the construction of the known headrest height adjustment devices described in DE 196 31 843 A1 and DE198 11 688 C1 where a biasing force of the spring is used for achieving either an upward or downward operation of the headrest, there is a danger that, a desired operation may not be achievable because of inadequate biasing force resulting from dispersion of sliding resistance in circumstances where the headrest height adjustment device is manufactured on the basis of large scale production.
A need thus exists for a headrest height adjustment device in which increases in space required for installment can be kept under control and the headrest operated stably in an upward and downward direction, in conjunction with a sliding movement of a seat body.